Cooling towers are used to cool liquid by contact with air. Many cooling towers are of the counter-flow type, in which the warm liquid is allowed to flow downwardly through the tower and a counter current flow of air is drawn by various means upward through the falling liquid to cool the liquid. Other designs utilize a cross-flow of air, and forced air systems. A common application for liquid cooling towers is for cooling water to dissipate waste heat in electrical generating and process plants and industrial and institutional air-conditioning systems.
Most cooling towers include a tower structure. This structural assembly is provided to support dead and live loads, including air moving equipment such as a fan, motor, gearbox, drive shaft or coupling, liquid distribution equipment such as distribution headers and spray nozzles and heat transfer surface media such as a fill assembly. The fill assembly material generally has spaces through which the liquid flows downwardly and the air flows upwardly to provide heat and mass transfer between the liquid and the air. One well-known type of fill material used by Ceramic Cooling Towers of Fort Worth, Tex. consists of stacked layers of open-celled clay tiles. This fill material can weigh 60,000 to 70,000 pounds for a conventional size air conditioning cooling tower. Structural parts of a cooling tower must not only support the weight of the fill material but must also resist wind forces or loads and should be designed to withstand earthquake loads.
Due to the corrosive nature of the great volumes of air and water drawn through such cooling towers, it has been the past practice to either assemble such cooling towers of stainless steel or galvanized and coated metal, or for larger field assembled towers, to construct such cooling towers of wood, which is chemically treated under pressure, or concrete at least for the structural parts of the tower.
Metal parts of cooling towers can be corroded by the local atmosphere or the liquid that is being cooled, depending on the actual metal used and the coating material used to protect the metal. Further, such metal towers are usually limited in size and are also somewhat expensive, especially in very large applications such as to cool water from an electric power generating station condenser.
Concrete is very durable, but towers made of concrete are expensive and heavy. Many cooling towers are located on roofs of buildings, and the weight of a concrete cooling tower can present building design problems.
Plastic parts are resistant to corrosion, but plastic parts ordinarily would not provide enough strength to support the fill material and the weight of the tower itself.
Wood has been used for the structural parts of cooling towers, but also has its disadvantages. Wood towers may require expensive fire protection systems. The wood may decay under the constant exposure not only to the environment, but also to the hot water being cooled in the tower. Wood that has been chemically treated to increase the useful life may have environmental disadvantages: the chemical treatment may leach from the wood into the water being cooled. Fiber reinforced plastic has been used as a successful design alternative to wood and metal.
To withstand expected lateral wind and seismic loads, support towers have generally been of two types: shear wall frame structures and laterally braced frame structures. Shear wall frame structures are generally of fiber reinforced plastic or concrete construction, and have a network of interconnected columns and beams. Shear walls are used to provide lateral resistance to wind and earthquake loads. In laterally braced framing structures, the cooling towers are generally made of wood or fiber reinforced plastic beams and columns, framed conventionally for dead load support; diagonal braces are used to resist lateral loads. The joints where the beams and columns meet are designed to allow for rotation between the structural elements. The joints do not provide lateral resistance to loading or racking of the structure.
Prior art solutions using fiber reinforced plastic include those shown in U.S. Pat. No. 5,236,625 to Bardo et al. (1993) and U.S. Pat. No. 5,028,357 (1991) to Bardo. Both patents disclose structures suitable for cooling towers, but a need remains for a mid-priced structure suitable for use as a cooling tower.
Thus, whitle prior fiber reinforced plastic tower structures have solved many of the problems associated with wood and metal cooling tower structures, many of the solutions to the problem of resistance to lateral loading have increased the costs of these units. Both the shear wall and laterally braced frames can be labor intensive to build, since there are many parts and many connections to be made. There are a large number of key structural elements, with more complex manufacturing and inventorying of parts, increasing the complexity of construction, and therefore the costs. And while the increased costs can be justified in many instances, a need remains for a lower cost cooling tower structure, and for lower cost cooling tower structures that meet less exacting design criteria where the prior structures go beyond the need.
In fiber reinforced plastic frame structures, one difficulty with the joint between the columns and beams has been that when made with conventional bolts or screws, the beams and columns can rotate with respect to each other. If tighter connections were attempted to be made with conventional bolts or screws, to limit rotation and provide lateral stability without adding diagonal bracing, the fiber reinforced plastic material could be damaged, and the problem worsened as the connecting members degrade the fiber reinforced plastic and enlarge the holes in which they are received.